1. Field of the Invention
This invention relates to radio communication systems and methods, and more particularly relates to interference cancellation systems and methods for minimizing or eliminating interference in radio receivers due to unwanted signals. Even more specifically, this invention relates to a cancellation system and method for situations where two or more interfering signals are received at the auxiliary or reference antenna with differing phases, relative to the receiver antenna.
2. Description of the Prior Art
FIG. 1 is a functional diagram of a conventional interference cancellation system connected to a radio receiver system, shown generally as including a receiver antenna 2, a receiver 4 and a transmission line 6 interconnecting the receiver antenna 2 and the receiver 4. The interference cancellation system includes an auxiliary antenna 8, the purpose of which is to receive an RF sample of an interfering signal and to provide a reference signal for the interference cancellation system. This reference signal is used to detect the presence, amplitude and phase of the same interfering signal in the transmission line 6 between the receiver antenna 2 and the receiver 4, in the following manner.
A first coupler 10 is electrically connected to the auxiliary antenna transmission line 12 to provide a portion of the reference signal corresponding to the interfering signal received by the auxiliary antenna 8 to one input of a synchronous detector 14. A sample of the signal received on the receiver transmission line 6 is provided to a second input of the synchronous detector 14 by using a second coupler 16 connected to the receiver transmission line 6. The synchronous detector 14 thus compares a portion of the reference signal and the sample signal from the receiver transmission line 6, and provides output signals which vary in accordance with the differences and similarities in phase and amplitude of the reference and sample signals.
The output signals of the synchronous detector 14 are modified by respective integrators/amplifiers 18 to provide control signals which are provided to a signal controller 20. The reference signal is also provided, through an appropriate amplifier 22, to the signal controller 20. The signal controller 20 and the synchronous detector 14 thus define an adaptive control loop in the interference cancellation system such that the signal controller 20, driven by the control signals, adjusts the amplitude and phase of the reference signal and provides an adjusted cancellation signal. The cancellation signal is then injected into the receiver signal path defined by the receiver antenna 2, transmission line 6 and receiver 4 with equal amplitude but in a phase which is opposite to that of the interference signal, thereby cancelling the interfering signal in the receiver path. A third coupler, which is referred to as a subtractor 24 in FIG. 1, is used to inject the cancellation signal into the receiver signal path. The interference cancellation system automatically and continuously maintains the amplitude and phase of the cancellation signal for maximum cancellation.
When the direction of the desired signal is fixed, a directive antenna may be used for the receiver antenna. If the direction of the interference signal is arbitrary, then an omni-directional antenna is generally used for the auxiliary antenna 8.
In a conventional interference cancellation system, such as the type described above and illustrated by FIG. 1 of the drawings, the strongest signal received at the auxiliary antenna 8, be it the desired signal or the interfering signal, is cancelled since it dominates and controls the outputs of the synchronous detector 14. When the interfering signal arrives in the same direction as the signal of interest, both the interfering signal as well as the desired signal are cancelled. When only the desired signal is present, or when the desired signal is stronger than the interfering signal, the conventional interference cancellation system must be disabled in order to prevent cancellation of the desired signal.
Another disadvantage of the conventional interference cancellation system is that the auxiliary antenna 8 and the receiver antenna 2 must be spaced apart from one another so that there is a phase difference between the reference signal from the auxiliary antenna and the sample signal taken from the receiver antenna. This phase difference is necessary so that the adaptive control loop of the cancellation system, and in the particular the synchronous detector 14 of the loop, can distinguish between the two signals and provide a proper detector output signal to the signal controller 20.